Predation

To feed on another living thing is to be a predator, to partake in predation. Even herbivores are predators of sorts, although by predators we usually mean those animals that kill and consume animal prey relatively quickly. These are the true predators, and often dwell at, or near, the top of the food chain.

The food chain in the forest begins with the sun's energy, arriving fresh from its 93,000,000 mile, eight minute journey across the Solar System. Most plants use photosynthesis to convert this solar energy to sugars, while extracting nutrients and water from the soil, and carbon dioxide from the atmosphere. Herbivores eat the plants, and carnivores then eat the herbivores. These may then be eaten by other carnivores. Each stage in the chain is known as a trophic level (from the Greek Trophikos: food). When the top predator dies, the nutrients are recycled back into the soil.

Many animals are both predator and prey, and are positioned part way along the food chain. The red squirrel (Sciurus vulgaris) is prey to animals such as the pine marten (Martes martes), but the squirrel itself will supplement its mainly herbivorous diet with insects and young birds in the nest and so is also a predator.

Animals at the very top of the food chain are the top predators. The lynx (Lynx lynx), the wolf (Canis lupus) and the bear (Ursus arctos) (which have all been exterminated in Britain), and a number of birds of prey are the main ones in the Caledonian Forest, and they generally have, or had, few threats other than humans. As we will see, the importance of predation in an ecosystem cannot be overstated.

Physical adaptations to hunting

Over millions of year, predation has ensured that life-forms evolve and adapt to become more efficient at catching their prey, or eluding their predators, and there is a truly phenomenal range of adaptations to meet these ends. Take the goshawk (Accipiter gentilis) for instance. In common with the other true hawks, this awesome bird has relatively compact wings. It is a woodland hunter, and this adaptation gives it astonishing manoeuvrability as it speeds through even fairly dense thickets after a bird or squirrel. Various species of dragonfly are also expert fliers, and can switch to virtually any direction in the blink of an eye, making them extremely effective aerial hunters.

Predators also have the tools required for killing and eating prey. Among these, 'sharp bits' feature prominently! For example, a golden eagle's (Aquila chrysaetos) sharp talons and strong grip are able to pierce and crush its prey to death. Cats such as the lynx jump on their prey's back and deliver a precise bite to the throat.

Venom is another method used. The adder (Vipera berus) kills prey such as lizards and small mammals with venom injected via its fangs. Many species of spider also inject venom into their insect prey, paralysing them and dissolving them from within.

Sensory adaptations

The European lynx serves as a good illustration of how a predator's senses are adapted to increase its hunting success. It has exceptionally acute vision that can detect movement 4 km away and it can hear high-pitched sounds as far as 5 km distant. Most predators have forward facing eyes (as humans do). This gives us binocular vision, and the ability to accurately judge distances - an important requirement for hunting. Contrast this with prey species such as deer, hares or many birds, which have eyes positioned further round on the sides of their heads, giving them a much wider field of vision, and enabling them to spot approaching danger more effectively.

Hunting strategies

There are many strategies for hunting prey. Some animals are ambush hunters. The sparrowhawk (Accipiter nisus) is one example, and will often watch its prey's activities for quite a while before making its surprise attack. Spiders have strategies at different ends of a scale. The familiar garden spider (Araneus diadematus) seems to embody stillness and patience as it waits at the centre of its orb web for a hapless insect to become ensnared. Wolf spiders such as Pardosa saltans on the other hand, are aptly named. Like their namesake they chase down their prey on foot.

Lynx and wolf also have contrasting strategies: the lynx is another solo ambush hunter, and speedily covers short distances. Wolves on the other hand work as a team, and often subject deer to a trial of stamina, sometimes chasing them for many miles to pick off the weakest individuals.

Defence against predators

Being a predator can be a high-risk occupation; not only can they face starvation when prey is scarce, but many prey animals are far from defenceless. Millions of years of predation have forced prey species to develop the ability to fight or flee.

Some animals have different types of physical protection. Horns, hooves and teeth can all be used in retaliation against a predator. The wild boar (Sus scrofa) has sharp tusks which it would use if cornered by a wolf. Even fairly minor injuries can have an impact on a predator's hunting ability, and can sometimes make the difference between life and death.

Some prey species use speed as their ally: the mountain hare (Lepus timidus), for example, can reach speeds of up to 50 kilometres per hour. Others use stillness: studies have found that when they are stalked by a weasel (Mustela nivalis), voles (Agrestis spp.) that 'freeze' are less likely to be caught than those that panic and run. Hiding is a time-honoured way of avoiding being eaten, and small mammals make use of holes to escape from predators. Others such as squirrels take refuge in trees, and birds to the air to escape.

Size can also be advantageous: the wild cattle known as the aurochs (Bos primigenius) stood about 1.8 metres (6 feet) at the shoulder, making the adults virtually invulnerable to predation. Even so, one of the deadliest of all predators, humans, finally drove it to extinction.

A number of birds and mammals have alarm calls that warn others nearby that a predator is in the area, increasing the chances of escape. Interestingly, blue tits (Parus caeruleus) for example, have a high-pitched alarm call that is above the hearing range of sparrowhawks, thus avoiding putting themselves at risk of being detected and eaten. Listening for alarm calls is a reliable way of spotting birds of prey, foxes or other predators.

Some birds will also mob a predator, congregating in larger numbers to drive it away. In the Caledonian Forest and elsewhere, it is not uncommon to see crows chasing a harassed-looking buzzard (Buteo buteo) away from the area.

Both predators and prey make use of camouflage to improve their chances of getting a meal, or of not becoming one. A roe deer fawn is born with a spotted coat which enables it to blend in to the forest floor when its mother goes off in search of food. Similarly the lynx has some spots as well as extreme stealth, enabling it to move virtually undetected.

In contrast wasps and bees often have characteristically visible markings, warning potential predators how inedible they are!

The effects of predation on prey

Predation obviously has an effect on the individual prey in the short term, and on prey populations over longer periods. Wolves keep deer herds healthy by culling sick and weak animals, the old and the young. They pose much less of a risk to a healthy, vigorous stag for example. In this way, only the strongest animals pass on their genes. In many cases predation prevents a prey species from becoming too abundant.

Other effects of predation are more subtle and complex. The prey species itself can even benefit, as lower numbers can mean less competition for food. However, while being hunted, prey animals will not be looking for food, which can affect their growth rate, reproduction and the like. Predator numbers themselves can also be influenced by sudden growth or declines in prey populations, which are affected by unexpected changes in food availability or by severe weather.

On a much larger timescale, predators and their prey each have a key influence on the evolution of the other. This is an ongoing process known as co-evolution. For example, the ancestors of today's red deer would have developed greater speed and stamina to escape from predators such as the wolf. A deer that was particularly fast and strong would be more likely to survive and reproduce, passing on genes for speed or strength. As a result, only the fastest, or most efficient predators would succeed in catching their prey. Again, the successful predator has more chance of passing on genes for speed, acute senses or whatever feature made it a successful hunter.

Wider ecosystem impacts

The loss of our top predators in the Caledonian Forest has given rise to a severely dysfunctional forest ecosystem. Because these species have been missing for centuries, we have almost become accustomed to a degraded biological community. However, by observing the effects of these predators in other temperate forest ecosystems, we can get a clearer picture of the complex role predation plays in keeping forests healthy.

Predation has a top-down influence on an ecosystem, with the effects reaching through several trophic levels. This is known as a 'trophic cascade'. Predators regulate herbivore numbers and influence their behaviour, which in turn affects the habitat itself. For example, anecdotal evidence from eastern European forests suggests that where wolf dens are present, the surrounding area will often have much lower herbivore pressure, resulting in denser woodland. In addition, carcasses left by predators can provide food for other species such as bears, eagles and carrion beetles (Nicrophorus spp.).

In Yellowstone National Park in the USA, the presence of wolves has had positive effects on the ecosystem. They were hunted to extinction there by the 1920s but were reintroduced in 1995. Since their return, there have been noticeable effects on the riparian (riverside) vegetation, with regeneration of quaking aspen (Populus tremuloides) willow (Salix spp.), and other species. This is because the elk (the American name for red deer, Cervus elaphus) have become wary about grazing down by the rivers, where their view is limited; this forces them onto higher ground. The aspen encourages beavers (Castor canadensis), while the beavers' dams attract moose (Alces alces), which like to graze in the pools, along with other wildlife. Ecologists noted that before the reintroduction, the control of elk numbers by humans was not enough to prevent aspen from being overgrazed. The reason for this was that even at lower numbers the elk were able to congregate densely enough to deplete what is one of their preferred food sources.

A study in Banff National Park in the Canadian Rockies revealed similar effects by comparing areas where wolves had naturally recolonised, with those in which they were partially excluded by high human activity. Elk numbers were ten times higher in the low-wolf areas, with higher recruitment of young animals. Again willow and aspen were depleted, with a corresponding reduction in beaver and songbird numbers.

Interestingly, in other parts of the North America, wolf and elk numbers have both increased in each others' presence, suggesting that effects on distribution are in some cases more significant than control of numbers.

Removing our native top carnivores has had an imbalancing effect on our forest ecosystems. Deer numbers are extremely high due to loss of predators, winter feeding by humans, and milder winters. This has been a key factor in the degradation of the Caledonian Forest, as the resultant overgrazing by excessive deer numbers has prevented the natural regeneration of the trees.

As well as allowing trees and scrub to regenerate, the influence of predation by larger carnivores would allow ground flora to thrive, with the berries and flowers providing food for birds, mammals and insects, while the cover offered by vegetation would benefit small mammals. Large predators also keep numbers of medium-sized predators such as foxes (Vulpes vulpes) in balance. This in turn reduces the pressure that foxes can have on ground-nesting birds such as the capercaillie (Tetrao urogallus).

Restoring predators to the Caledonian Forest

The restoration of missing predators is a controversial idea that has been the subject of considerable debate in recent years. Among the complex issues to be addressed are the potential conflicts with farming interests, ensuring the presence of suitable habitat, and fundamentally, tackling the deep-seated misconceptions that many people have about predator behaviour. Close involvement of farmers and the public is essential, and we can learn from other countries that have already carried out reintroductions.

While it may be some time before we could realistically hope to see animals such as the wolf back in their rightful place, the lynx is a much more feasible candidate for the nearer future, as there is certainly sufficient habitat and prey. It is clear from the scientific evidence that a carefully planned, and appropriately timed reintroduction of our missing predators is essential for the long-term restoration of a healthy forest ecosystem.

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About Trees for Life

The core purpose and vision of Trees for Life is to restore the Caledonian Forest and all its constituent species of flora and fauna to the Scottish Highlands. By engaging volunteers to achieve this, we aim to provide a powerful and educational experience that will promote the work of restoration to wider audiences. Through providing a demonstration of the techniques of ecological restoration, we aim to inspire similar projects elsewhere in the UK and around the world.